1. Field of the Invention
The present invention relates to a manufacturing method for a thin film diode (TFD) in a liquid crystal display (LCD) device.
In the liquid crystal display device (LCD) having a structure where a liquid crystal is sealed between two glass substrates, to display a specified image by driving the liquid crystal, a thin film diode (TFD) serving as an active element (switching element) is arranged between a data line formed on an under glass substrate and each of the drive electrodes arranged in a dot matrix fashion. With such structure, interference between pixels can be prevented, thereby enhancing a quality of the displayed image. The present invention relates to a manufacturing method for such a thin film diode.
2. Description of the Prior Art
The structure of a liquid crystal display device to which the present invention is applied will be described with reference to FIGS. 1 to 3. First, with reference to FIG. 1, the total constitution of the color liquid crystal display will be described.
In FIG. 1, reference numeral 1 denotes an under glass substrate, and reference numeral 11 denotes an upper glass substrate. A stripe-shaped data line 12 and a plurality of drive electrodes 13 arranged in a dot matrix fashion are formed on the lower glass substrate 1. A thin film diode (TFD) 8 serving as an active element (switching element) is disposed between the data line 12 and each of the drive electrodes 13.
An orienting film 15 made of a polyimide resin film to orient liquid crystals is disposed on the data line 12, the drive electrodes 13, and the thin film diode 8.
On the other hand, a color filter 17 composed of color filter elements is disposed under the surface of the upper glass substrate 11. A black matrix 16 is formed in the boundary regions of the color filter elements. The red (R), green (G), and blue (B) color filter elements are arranged so as to correspond to each of the drive electrodes 13 on the glass substrate 1, respectively.
A stripe-shaped scanning electrode (not shown) is disposed under the surface of the color filter 17 interposing an insulating film (not shown). The scanning electrode is in a perpendicular to the data line 12. Further, an orienting film 18 made of a polyimide film serving for orienting the liquid crystal is disposed on the scanning electrode.
A liquid crystal (not shown) is sealed between the orientating films 15 and 18. Further, polarizing plate 19 and 20 are arranged on each outer surface of the lower and upper glass substrate 1 and 11 respectively so that the polarized axises thereof are perpendicular to each other. In the liquid crystal display device described above, if the color filter 17 is omitted, the liquid crystal display dedive functions as a black/white liquid crystal display device.
Next, referring to FIG. 2 and FIG. 3, a structure of the thin film diode 8 incorporated in the liquid crystal display device of the present invention will be described.
FIG. 3 is a sectional view taken along the line III--III shown in a plan view of FIG. 2.
The thin film diode 8 has a Metal-Insulator-Metal structure composed of a lower layer film 2 protruding from the data line 12, an insulating film 3 formed on the surface of the film 2, and an upper layer film 4 formed on the film 3.
The upper layer film (electrode) (hereinafter referred to as upper layer film) 4 serves as a part of the drive electrode 13.
Specifically, a punched portion 13a is formed in the drive electrode 13 close to the portion thereof which crosses the lower layer film (electrode)(hereinafter referred to as lower layer film) 2, so that the thin film diode 8 presents a plan view pattern in which the lower and upper layer films 2 and 4 cross.
A manufacturing method of the thin film diode 8 will be briefly described below.
A tantalum (Ta) film as a material for the data line 12 and the lower layer film 2 is formed on the glass substrate 1. The tantalum (Ta) film is patterned by means of a photo-etching processing. Thereafter, an anodic oxidation processing is carried out using the patterned tantalum (Ta) film as an anode to form the insulating film 3 made of a tantalum pentoside (Ta.sub.2 O.sub.5) on the tantalum (Ta) film. This anodic oxidation processing is performed in such a manner that citric acid, the tantalum film, and platinum (Pt) are used as an anodic oxidation liquid, an anode, and a cathode, respectively, and a direct current is applied between the tantalum film and platinum.
Thereafter, indium-tin-oxide (ITO) film serving as a material for the drive electrode 13 and the upper layer film 4 is formed on the entire surface. Then, the indium oxide tin film is subjected to patterning by means of a photo-etching processing so that the upper layer film 4 and the drive electrode 13 are formed.
The conventional constitution and manufacturing method thereof of a thin film diode 8 in such a liquid crystal display device will be further described with reference to FIGS. 4 to 7.
The vertical sectional view of the lower layer film 2 constituting the thin film diode on the glass substrate 1 presents either a rectangular shape as shown in FIG. 4 or a tapered-trapezoidal shape as shown in FIG. 5.
The reason why it presents such the shape is that patterning of the film 2 is ordinarily carried out by one etching process.
Specifically, in a manufacturing method for such a thin film diode, the substance for the lower layer film is initially formed on the upper surface of the glass substrate 1 by either a sputtering technique or a chemical vapor deposition technique. Subsequently, a resist pattern is formed on the substance for the lower layer film by using a lithography technique.
Thereafter, an etching processing by means of either a wet etching technique or a dry etching technique is carried out using the patterned resist as a mask to form a pattern of the lower layer film 2 as shown in FIG. 4 or FIG. 5.
When etching the lower layer film, it is difficult to control the etching shape by the wet etching technique. However, with the dry etching technique, it is possible to produce the tapered-trapezoidal shape shown in FIG. 5 by controlling an etching speed ratio of the resist to the substance for the lower layer film.
Subsequently, as shown in FIG. 6 or FIG. 7, an anodic oxidation process using the lower layer film 2 as an anode is performed to form the insulating film 3 on the surface of the film 2.
Further, a substance for the upper layer film is formed on the insulating film 3 by either the sputtering technique or the chemical vapor deposition technique. Thereafter, a resist pattern (not shown) is formed on the substance for the upper layer film by the lithography technique.
Following the above processes, an etching process to form the pattern of the upper layer film 4 is carried out by either the wet etching technique or the dry etching technique using the patterned resist as a mask. As a result, the thin film diode 8 of the Metal-Insulator-Metal structure is completed.
At the time of the formation of the thin film diode, it is essential to form a region where the lower and upper layer films 2 and 4 overlap each other. The upper layer film 4 overlapping with the lower layer film 2 sometimes breaks at its step portion. The situation of the breaking of the film 4 will be described with reference to FIGS. 6 and 7.
Referring to FIG. 6, the region where the upper layer film 4 is formed stretches over the glass substrate 1 and the insulating film 3.
Therefore, the film property of the film 4 on the insulating film 3 is different from that on the glass substrate 1 due to the difference of the materials of the substrate 1 and the film 3. Specifically, the boundary due to the difference of the film natures is produced between the glass substrate 1 and the insulating film 3. Further, a growth direction of the covering film serving as the upper layer film 4 on the etched side region of the glass substrate 1 is different from that thereof on the etched side region of the lower layer film 2. As a result, both growth directions meet with each other at the boundary so that a crystallinity of the film 4 is deteriorated at the boundary.
Therefore, the crystallinity of the covering film serving as the upper layer film 4 is deteriorated in addition to the difference of the film property at the boundary region (shown by the arrow A) between the glass substrate 1 and the insulating film 4. Thus, the portion of the covering film at the boundary region is more liable to break than other portions thereof. The occurrence of such breaking of the film produces a defective pixel, leading to a decrease in a quality of the liquid crystal display device.
Especially, when the surface of the lower layer film 2 is oxidized by the anodic oxidizing technique to form the insulating film 3 and the patterned upper layer film 4 formed on the film 3, the film 3 is formed only on the surface of the lower layer film 2. Thus, the upper layer film 4 is formed stretching over the glass substrate 1 and the insulating film 3 which have different natures. This causes severe problems.
Moreover, the insulating film 3 grows isotopically during the formation thereof using the anodic oxidation technique. Accordingly, though the section of the lower layer film 2 formed on the glass substrate 1 is made into a taper-shaped trapezoid by etching as shown in FIG. 5, the section of the insulating film 3 presents the shape shown in FIG. 7. The portion of the insulating film 3 at the boundary between the glass substrate 1 and the film 3 does not present the tapered shape so that the effects of the tapered shape is decreased. For this reason, the portion of the upper layer film 4 at the boundary (shown with an arrow B) between the glass substrate 1 and the insulating film 3 is also apt to be broken.